Fixes case when site prefix != lang code

There are several wikipedias whose site prefix doesn't equal site language code. For these, the tool would fail to fetch labels from Wikidata, as seen here:

http://tools.wmflabs.org/not-in-the-other-language/?lang1=no&proj1=wiki&lang2=et&proj2=wiki&cat=Estland&depth=2&starts_with=&doit=Do+it

It's possible that the function should be moved to the 'magnustools' repo if needed by more tools.